1. Field of the Invention
The present invention relates to a circuit for generating a reference voltage for use by other circuits, and more specifically to a circuit for generating a reference voltage for realizing a xe2x80x9csoft startxe2x80x9d of various circuits.
2. Description of the Related Art
There are electric circuits and electronic circuits that must be operated based on an externally applied reference voltage. For example, a DC/DC converter is normally supplied with a reference voltage which specifies an output voltage to be maintained.
A reference voltage must not be effected by temperature changes and so on. It is also necessary for a reference voltage to be increased gradually from 0 volts to a target voltage at the activation of an electric circuit or electronic circuit to which the reference voltage is applied. The process of gradually increasing a voltage from 0 to a target value under a predetermined condition can be referred to as a xe2x80x9csoft startingxe2x80x9d process, and a circuit for realizing the process can be referred to as a xe2x80x9csoft start circuitxe2x80x9d.
FIG. 1A shows an example of a reference voltage circuit provided with a soft starting function. A constant voltage source 101 is, for example, a band gap reference circuit, and outputs a constant voltage Vbgr independent of a temperature. The reference voltage Vref matches the voltage Vbgr except at initial operation. A constant current source 102 supplies a current Iconst to charge a capacitor Css.
FIG. 2A shows a soft starting process performed by the reference voltage circuit shown in FIG. 1A. In FIG. 2A, it is assumed that a switch SW of FIG. 1A is OFF at initial state.
When a power supply is started at time T1, then a capacitor Css is charged by the constant current source 102 according to which a reference voltage Vref increases linearly. When the reference voltage Vref increases up to the level of the voltage Vbgr output from the constant voltage source 101 and the switch SW is turned on at time T2, the voltage Vbgr is output as the reference voltage Vref.
The above described reference voltage Vref is used, for example, as a parameter which indicates an output voltage of a DC/DC converter. In this case, the output voltage of the DC/DC converter changes in accordance with the reference voltage Vref. Therefore, if the reference voltage Vref as shown in FIG. 2A is applied to the DC/DC converter, the output voltage of the DC/DC converter gradually increases in accordance with the change in the reference voltage Vref.
In the soft starting process performed by the reference voltage circuit shown in FIG. 1A, the switch SW is turned on at time T2, as described above. Here, time T2 is the timing when the reference voltage Vref reaches the voltage Vbgr.
However, it is not easy to turn on the switch SW accurately at time T2. For example, if the switching timing is delayed and the switch SW is turned on at time T3 as shown in FIG. 2B, the reference voltage Vref temporarily exceeds the voltage Vbgr. That is to say, an overshoot occurs. On the other hand, if the switch SW is turned on before time T2 (at time T4), the reference voltage Vref temporarily drops before it reaches the voltage Vbgr as shown in FIG. 2C, or rapidly increases as shown in FIG. 2D. FIGS. 2C and 2D are examples of cases where the current supply/absorb ability of the constant voltage source 101 is low and high, respectively.
When the reference voltage Vref does not smoothly increase, the operation of the circuit using the reference voltage Vref becomes unstable. For example, in a case where the output voltage of the DC/DC converter is determined in accordance with the reference voltage Vref, the output voltage of the DC/DC converter becomes unstable if the voltages shown in FIGS. 2B through 2D are applied.
A reference voltage circuit with the soft starting process can be realized with the simple circuit shown in FIG. 1B. With this circuit, the above described problem caused by an inaccurate switching timing of the switch SW does not occur. However, with this circuit, the reference voltage Vref does not increase linearly, and a time period taken for the soft starting process cannot be accurately defined. In addition, if a resistance of the resistor R is large, the impedance of the Vref terminal becomes high, and the circuit operation is easily affected by the external noise.
The present invention intends to solve the above described problem, and aims at providing a reference voltage circuit capable of realizing a stable soft starting process.
The reference voltage circuit according to the present invention includes a constant voltage source, and a voltage generated by the constant voltage source is output as a reference voltage to be used by other circuits. The reference voltage circuit includes an output terminal, a current source, a capacitor connected to the output terminal and charged by the current source, a comparator for generating an instruction signal when the voltage at the output terminal exceeds a threshold voltage, which is lower by a predetermined value than the voltage generated by the constant voltage source, and a switch for connecting the constant voltage source to the output terminal through a resistor when the instruction signal is generated.
The output voltage from the reference voltage circuit increases in two steps up to a target voltage (voltage generated by the constant voltage source). That is to say, the output voltage from the reference voltage circuit increases according to the current source in the first step, and then increases along the RC curve depending on the resistance value of the resistor and the capacity of the capacitor in the second step. Therefore, the output voltage of the reference voltage circuit smoothly increases, without an abrupt change, until it reaches the target voltage.